The present invention is in the technical field of papermaking, and, more particularly, in the technical field of wet-end additives to the papermaking stock or furnish. In particular the invention relates to a papermaking stock, a method for increasing or enhancing the retention of components of a papermaking stock during the manufacture of paper, and a method of producing paper. In an especially important embodiment the methods are carried out in relatively xe2x80x9cclosedxe2x80x9d mill water systems while simultaneously increasing drainage and decreasing the amount of deposits from colloidal hydrophobic particles often referred to as xe2x80x9cstickiesxe2x80x9d or xe2x80x9cpitchxe2x80x9d on the paper machine.
The manufacture of paper is a complex process which can be broken down into a series of less involved processes. One of the more important processes occurs at the paper machine. At this location, an aqueous cellulosic suspension, stock, furnish or slurry is formed into a paper sheet. The cellulosic suspension is made by providing a thick stock, diluting the thick stock to form a thin stock, draining the thin stock on a forming fabric to form a sheet, and drying the sheet.
The cellulosic slurry is generally diluted to a known consistency (based on percent dry weight of solids in the slurry) of less than 2 percent. Ideally, the consistency is between 0.8 and 1.5 percent.
The cellulosic slurry is generally, but not necessarily, a mixture of chemical, mechanical and secondary (e.g., deinked) pulps. For example, this includes all paper and board furnishes based on mechanical pulp and, in part, semi-bleached kraft pulp, unbleached kraft pulp, and/or unbleached sulfite pulp. The mechanical pulps may be stone-groundwood, pressure groundwood, thermomechanical pulp, or semi-chemical mechanical pulp. Other pulps may include deinked pulps, reslushed newsprint or any secondary fiber source.
Cellulosic slurries of high quality pulps can also be used to produce fine paper grades (e.g., photocopying paper), tissue or toweling sheets. These slurries include highly bleached mechanical or chemical pulps.
It is common to include various inorganic materials, such as bentonite and alum, and/or organic materials, such as various natural, modified natural, or synthetic polymers in the thin or thick stock for the purpose of improving the drainage and retention processes.
Such materials can be added for diverse purposes such as, for example, pitch control, increased drainage and retention, improved formation, increased wet and dry strength, defoaming, facilitation of release from drying rolls, and decolorization of effluents.
In addition, many grades of paper include substantial levels of inorganic fillers such as, for example, kaolinite, calcium carbonate, and titanium dioxide. The percentage of mineral filler added to a papermaking slurry may vary between 0 and 35% by weight of dry paper depending on the type of sheet being formed.
In the papermaking process, much of the pulp is separated from the fibers, fillers, and pigments by filtration. The filtrate, which is called the white water, contains a large amount of unretained colloidal particles which may be fibre fragments, mineral fillers, deinking plant materials, or pigment particles. The poor retention of these is a consequence of the difficulty in the filtration of material characterized by colloidal or nearly colloidal dimensions. Poor fines retention is a serious problem because it results in the loss of valuable cellulosic material and the additional loading of water treatment facilities.
The least expensive and oldest dewatering method is simple gravity drainage. More expensive methods which are also used include vacuum, pressing, and evaporation. Drainage may be accomplished either horizontally or vertically, by one side of the forming sheet only or by both sides.
In practice, a combination of such methods is employed to dewater or dry the sheet to the desired water content. Since drainage is the first dewatering method and the least expensive, improvement in the efficiency of drainage will decrease the amount of water required to be removed by other more costly methods such as drying. This will improve the overall efficiency of the process.
The papermaking fibers employed in papermaking are often of low grade and are predominantly of the mechanical type and include groundwood, thermomechanical pulp, deinked secondary fibers, semi-chemical pulps, and semi-bleached chemical kraft pulps. The cellulosic fibers thus produced are rarely very xe2x80x9ccleanxe2x80x9d and are rarely completely separated from the residual process liquors which contain substantial levels of both organic and inorganic impurities. These impurities are derived from the pulping process and by-products which are naturally present in wood (Linhart F., Auhorn W. J., Degen H. J. and Lorz R., Tappi J. 70(10) 79-85 (1987), Sunberg K., Thornton J., Pettersson C., Holmbom B., and Ekman R., J. Pulp Paper Sci., 20(11), J317-321 (1994)). These are often referred to as detrimental substances because they interfere with the function of many additives.
Detrimental substances increase the cationic demand of the pulp slurry. The cationic demand is the number of equivalents of cationic charge that has to be added to the slurry to neutralize the excess anionic charge of the pulp slurry. The cationic demand is usually met using a low molecular weight ( less than 500 000) highly charged synthetic cationic polyelectrolyte. These polymers are, for example, the following: polyethyleneimines, polyamines having a molecular weight of more than 50,000, polyamidoamines modified by grafting onto ethyleneimine, polyamidoamines, polyetheramines, polyvinylamines, modified polyvinylamines, polyalkylamines, polyvinylimidoazoles, polydiallydialkyl ammonium halides, in particular polydiallyldimethylammonium chloride. These polyelectrolytes are soluble in water and are used in the form of aqueous solutions.
The cationic demand of pulps used for making, for instance, newsprint is often above 1000 meq./mL of stock so that improvements only become significant with polymer weights of above 1000 grams dry polymer per tonne dry weight of paper. Such large amounts render treatment uneconomical.
Impurities in papermaking furnishes which need to be neutralized by the cationic polymer are present in solution as dispersed colloidal particles, and/or dissolved substances such as lignosulfonates and sulfites, kraft lignin, hemicelluloses, lignans, humic acids, dispersed wood resins, rosin acids and chemical by-products. These impurities impart a large negative charge on the surfaces of cellulose fibers and other materials when they are dispersed in water.
Recently, due to environmental legislation, the level of the aforementioned impurities in papermachine white-water systems has further increased. This increase is a consequence of the increased tendency for paper mill operations to xe2x80x9cclose upxe2x80x9d the paper machine white water systems and recycle as much white water as much as possible.
A second problem often associated with the manufacture of paper is the accumulation of wood resin and synthetic hydrophobic materials on the surfaces of the process equipment. Wood resin is usually defined as the material in wood which is insoluble in water, but soluble in organic solvents (Mutton, D. B., xe2x80x9cWood Extractive and Their Significance to the Pulp and Paper Industriesxe2x80x9d Chap. 10, Wood Resins, Ed. W. E. Hills, Academic Press, New York (1962)). The weight of wood resin from all species of trees consists usually of 1-5% based on total weight. From the teachings of U.S. Pat. No. 5,468,396 it is seen that increased reuse of mill white water causes a build-up in the concentration of water-borne resins (Allen L. H. and Maine C. J., Pulp Paper Can., 79(4): pp. 83-90 (1978)) and exacerbates the tendency for pitch deposition (Allen L. H., Tappi J., 63(2), pp. 82-87, (1980)). Many chemicals used to combat foam in pulp and paper mills end up dispersed in the aqueous phase of a pulp suspension and co-deposit with wood resin (Dorris G. M., Douek M., and Allen L. H., J. Pulp Paper Sci., 11(5): J149-154 (1985); Dunlop-Jones N. and Allen L. H., J. Pulp Paper Sci., 15(6): J235-241 (1989)). The presence of high amounts of dissolved and dispersed resin in paper machine process liquids usually also leads to reduced paper strength and runnability (Wearing, J. T., Ouchi, M. D., Mortimer, R. D., Kovacs, T. G., and Wong, A., J. Pulp Paper Sci., 10(6): J178 (1984)). Synthetic hydrophobic materials are usually introduced via deinked pulps and have similar chemical and physical properties to wood resins.
U.S. Pat. No. 5,468,396 teaches the use of a centrifugal deresination of the pulp and paper process liquids as an economical method to remove detrimental colloidal pitch. Furthermore U.S. Pat. Nos. 5,468,396 and 4,313,790 teach further prior art for reducing the concentrations of dissolved and dispersed resin which include the use of alum, dispersants, talc (Allen L. H., Tappi J., 63(2): pp. 82-87 (1980)); Douek M. and Allen L. H., J. Pulp Paper Sci., 17(5): J171-177 (1991)), sequestrants and a number of non-chemical methods such as bleeding the system, discarding of wash water, the use of a Frotapulper, followed by caustic extraction, as described by MoDo, and saveall flotation. Most of these methods are either too expensive under most circumstances or the practice is no longer tolerated.
In light of the aforementioned discussions, there has been ongoing extensive research into the development of new retention aids which increase retention and improve drainage in closed, highly contaminated systems. Traditional retention aids have had only a limited success in accomplishing these goals.
Increased retention and drainage allow significant economic benefits for a mill. Increased retention allows for cost savings in terms of reduced fibre consumption, cleaner machine operations, and decreased cost of effluent treatment. Increased drainage allows increased savings in terms of lower steam consumption brought about by a dryer sheet at the drying section.
In U.S. Pat. No. 4,313,790, inventors Pelton, Allen and Nugent have shown that a combination of kraft lignin or modified kraft lignin and poly(ethylene oxide) effectively increases fines retention and decreases pitch deposition on a papermaking machine in a papermaking process. A possible drawback to this system is the fact that mineral filler retention is not very high.
One method extensively used in the industry to improve the retention of cellulosic fines, mineral fillers, and other furnish components on the fiber mat is the use of a coagulant/flocculant dual polymer program system. The coagulant and flocculant are added ahead of the paper machine. In such a system a low molecular weight (usually  less than 500, 000), highly charged polyelectrolyte coagulant or cationically modified starch is added first to the furnish. This has the effect of reducing the cationic demand of the furnish and reducing the negative surface charges present on the particles in the furnish. This initial addition of the coagulant accomplishes an initial degree of agglomeration and also tends to fixate mineral fillers and colloidal pitch/stickies to the fibers. The addition of the coagulant is then followed by the addition of the flocculant. Such flocculant is generally, but not necessarily, a high molecular weight anionic, cationic, or neutral synthetic polymer which bridges the particles or agglomerates. Such a combination increases drainage and retention.
Another system employed to provide an improved combination of retention and drainage is described in Canadian Patents 1,168,404 and 1,255,856 by inventors Langeley and Litchfield. The above patents describe the addition of bentonite prior to a high shear point followed by the addition of a cationic or anionic polymer after the shear point. The initial addition of bentonite is thought to absorb the detrimental substances present in solution. The shearing generally is provided by one or more stages of the papermaking process such as the centriscreening. At these shear points the shearing breaks down the large flocs formed prior to the shear point. This system is sold under the tradename Organosorb/Organopol.
Canadian Patents 1,322, 435 and 1,259,153 call for the addition of low molecular weight synthetic polyelectrolyte and/or high molecular weight cationic flocculant prior to a shear point followed by the addition of bentonite after the shear point. This system is often referred to as the Hydrocol system.
U.S. Pat. No. 4,749,444 by Lorz, Auhom, Linhart, and Matz teaches the addition of bentonite to a thick stock followed by the addition of a coagulant to the thin stock prior to a shear point and the subsequent addition of a high molecular weight cationic or anionic flocculant after the shear point.
The system described in U.S. Pat. No. 4,388,150 teaches the combination of cationic starch followed by colloidal silica to increase the amount of material retained in the sheet. Yet another variation of the system is described in U.S. Pat. Nos. 4,643,801 and 4,795,531 which use, in addition to starch, synthetic polymers.
Additional systems to improve drainage and retention have also been proposed. South African Patent 2 389/90 corresponding to U.S. Ser. No. 397,224 teaches the use of a single, high molecular weight cationic polymer.
U.S. Pat. No. 5,089,520 suggests a drainage and retention program in which a cellulose papermaking slurry is treated with a high molecular weight cationic (meth)acrylamide polymer prior to at least one shear stage followed by the addition of a low molecular weight anionic polymer at least one shear stage subsequent to the addition of the cationic polymer.
U.S. Pat. No. 5,266,164 by Novak and Fallon provides a method for improving the retention of mineral fillers and cellulose fibers on cellulose fiber sheet. This is accomplished by the addition of an effective amount of high molecular weight cationic polymer prior to a shear point followed by the addition of a high molecular weight anionic flocculant after the shear point. The difficulty with the use of the aforementioned chemistries in xe2x80x9cclosedxe2x80x9d mill systems is their loss of effectiveness as retention and drainage aids (Allen, L. H., Polverari, M., Levesque B., and Francis D. U., 1998 Tappi, Coating/Papermakers Conference, New Orleans, Book 1, pp. 497-513 (1998)). A further difficulty with retention aids is that some polymer chemistries work better in some mills and worse in others.
WO 95/03450 teaches the use of cationic multi armed star-like polymers (hereinafter referred to as CMA-PAM) as an effective component to improve the retention of fines fraction by structural characteristics of multi armed polymer chains connected with one starting point on the compound. The CMA-PAM were synthesized by using pentaerythritol triacrylate (PETA) as the starting point. The three acrylate bonds are then reacted with the monomers acrylamide (AM) and dimethylamino-ethylacrylate (DMAEA-MC). Ammonium persulfate (APS) was used as the initiator. The structure formed is said to be star-like because the linear DMAEA-MC-AM chains extend from the central starting point, PETA. Depending on the DMAEA-MC-AM ratios the viscosity of the CMA-PAM vary between 86 and 450 centipoise (cP) and the charge densities do not exceed 1.5 meq./g at pH=7. The star-like structure was found to be more resistant to shear than linear PAM.
WO 95/03450 is thus concerned with polyols as starting compound; linear AM and DMAEA-MC chains are xe2x80x9cattachedxe2x80x9d to the polyol OH groups. The maximum number of branches from the center is 4; these are not dendrimers. Dendrimers, while also starting from a central point, continue to xe2x80x9cbranch outxe2x80x9d with every subsequent reaction.
Agents are also added to some papers, during fabrication to improve the wet strength of the product paper; wet strength agents are generally required for requiring wet strength papers such as tissue and towel, but are not required for printing papers. The function of a wet strength agent is different from the function of agents for enhancing retention of papermaking stock components and of agents for increasing drainage and there is no correlation between these different agents employed for different functions in papermaking.
Thus melamine formaldehyde and urea formaldehyde are among the most commonly employed wet strength agents in paper manufacture but have no utility as retention aids.
It is an object of this invention to provide a method of enhancing retention of components of a papermaking stock in a cellulosic sheet formed from the stock.
It is a particular object of this invention to provide a method of producing paper employing a dendrimeric polymer to enhance retention of components of a papermaking stock in the paper formed from the stock.
It is still another object of the invention to provide a papermaking stock containing a dendrimeric polymer to enhance retention of papermaking components of the stock in a cellulosic sheet formed from the stock.
In accordance with one aspect of the invention there is provided a papermaking stock comprising: an aqueous paper-forming cellulosic dispersion of papermaking components comprising cellulosic papermaking fibers and papermaking additives in an aqueous vehicle, characterized in that said dispersion contains a dendrimeric polymer as an agent to enhance retention of said components in a cellulosic sheet formed from said dispersion in papermaking, and in an amount to effect such enhanced retention and provide a cellulosic sheet having an enhanced content of the papermaking components as compared with a cellulosic sheet from a corresponding aqueous paper-forming cellulosic dispersion of papermaking components free of said dendrimeric polymer, said dendrimeric polymer being capable of developing a positive charge at an operating pH of papermaking.
In accordance with another aspect of the invention there is provided a method of enhancing retention of components of a papermaking stock in a cellulosic sheet formed from said stock in papermaking, said stock comprising an aqueous paper-forming cellulosic dispersion of papermaking fibers and papermaking additives in an aqueous vehicle, characterized by the inclusion in said dispersion of a dendrimeric polymer being capable of developing a positive charge at an operating pH of papermaking in an amount to enhance retention of said components in the cellulosic sheet.
In accordance with still another aspect of the invention there is provided a method of producing paper comprising forming a cellulosic sheet from a papermaking stock comprising an aqueous paper-forming cellulosic dispersion of papermaking components comprising papermaking fibers and papermaking additives in an aqueous vehicle characterized by enhancing retention of said components in the cellulosic sheet by the enhancing method of the invention, recovering a cellulosic sheet from the stock having an enhanced content of the papermaking components as compared with a cellulosic sheet formed from a corresponding aqueous paper-forming cellulosic dispersion of papermaking components free of the dendrimeric polymer, and recovering an aqueous fraction of the stock having a diminished content of the papermaking components.
Still further the invention provides paper produced by the aforementioned process of the invention.
In still another aspect of the invention there is provided a cellulosic paper sheet derived from an aqueous paper-forming cellulosic dispersion of papermaking components and a dendrimeric polymer capable of developing a positive charge at an operating pH of papermaking, said paper sheet containing said dendrimeric polymer and having an elevated content of the papermaking components of the dispersion, as compared with a paper sheet derived from a corresponding dispersion free of said dendrimeric polymer.
In accordance with yet another aspect of the invention there is provided use of a dendrimeric polymer to enhance retention of components of a papermaking stock in a cellulosic sheet formed from the stock, said polymer being capable of developing a positive charge at an operating pH of papermaking.
Thus a process has been discovered for the increase or enhancement of fines and filler retention and a decrease of pitch and/or stickies deposition during the manufacture of paper or paperboard, which involves the addition to the papermaking suspension of a dendrimeric polymer typically as a polymer solution. This system has also shown itself to be effective in xe2x80x9cclosedxe2x80x9d mill systems.
Alternatively, the dendrimeric polymer may be added to the diluted filler slurry, prior to addition of the filler slurry to the paper stock, when producing filled grades or to the undiluted thick stocks, prior to dilution.
When the present invention is practiced, the retention of fines and filler is increased which in turn results in decreased fines in the white water which, in turn, facilitates a lower head box consistency, a higher headbox freeness, and a more even distribution of fines and filler in the cellulosic sheet. In addition, practise of this invention fixes dispersed wood resin and stickies in the cellulosic sheet and results in a decrease in problems due to pitch deposition on the paper machine.
Other benefits from the practice of this invention include increased drainage, increased white water reuse, increased closure, lower energy consumption, and increased fines retention.
Using this invention it is possible to make any grade of paper, for example newsprint, board, and the so-called groundwood specialty grades. Tissue, toweling, and other fine papers can also be produced by practising the invention.
Papers and paperboards may be produced using, as the principle raw material groundwood (GWD), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), pressurized groundwood (PGW), bleached kraft (BK), semi-bleached kraft (SBK), unbleached kraft (UBK), sulfite or sulfate pulps. Other suitable pulps such as deinked (DIP) and refiner mechanical pulp (RMP) may also be used. Each of these pulps may contain short or long fibers.
It is also possible to produce both filler free and filler containing papers. The maximum filler content of the paper is typically 40%, by weight, based on oven dried fiber but is generally 0 to 35%, by weight, and preferably between 5 to 15%, by weight. Examples of suitable fillers are clay, kaolin, chalk, talc, precipitated calcium carbonate (PCC), ground calcium carbonate (GCC), titanium dioxide, calcium sulfate, barium sulfate, alumina, satin white, organically synthesized fillers, or mixtures thereof.
A wet strength agent, for example, a melamine formaldehyde or a urea formaldehyde may be added to the papermaking stock, in addition to the dendrimeric polymer of the invention, especially in the case of papers requiring wet strength papers such as tissue and towel.
In most cases, however, especially in printing papers, no wet strength agent is required and the dendrimeric polymer is added to the papermaking stock without the addition of a wet strength agent.
In particular, the papermaking stock may be free of wet strength agents.
The dendrimeric polymer enhances retention of papermaking components in a cellulosic sheet formed from a cellulosic dispersion of papermaking components and produces a cellulosic sheet having an enhanced content of the papermaking components as compared with a cellulosic sheet formed from a second cellulosic dispersion which differs only in that it is free of the dendrimeric polymer.
On the other hand, an aqueous fraction of the papermaking stock of the invention separated from the cellulosic sheet formed from the stock, has a diminished content of papermaking components, as compared with an aqueous fraction separated from the aforementioned second cellulosic dispersion free of the dendrimeric polymer.
The term dendrimeric macromolecules is understood as embracing very generally highly branched macromolecules that emanate from a central core and are synthesized through a stepwise, repetitive reaction sequence. Dendrimeric macromolecules are often referred to as xe2x80x9cstarburstxe2x80x9d polymers. Dendrimers are a new class of macromolecules with a hyperbranched structure. This structure is well defined in terms of chemical composition and three-dimensional configuration. Dendrimers are synthesized in a stepwise manner, which provides unique control over chemical and physical properties. This control allows for the development of products which are tailored to specific applications. For example the end groups of the dendrimers are very well accessible for all kinds of modification reactions. Examples of modified end groups include carboxylic or fatty acid derivatives (Tomalia, D. A., Naylor, A. M., and Goddard, W. A., Angew. Chem. Intl. Ed. Engl., 29, 138-175 (1990); Frechet J. M., Science, 263, 1710-1715 (1994)).
Due to the repetitive reaction sequence in the synthetic procedure, dendrimers can be obtained with a chosen number of generations and end-groups. These structures are well defined in terms of both chemical composition and three dimensional configuration. Since dendrimers are synthesized in stages, one is afforded unique control over their chemical and physical properties such as size, shape, reactivity, solubility, and three dimensionality. This control allows the development of products which are tailored to specific applications. Reference is made to the following literature citing the synthesis of dendrimers: (Newkome G. R. et al., Macromolecules, 26(9), 2394-2396 (1993); Jansen et al., Science, 266, 1226-1229 (1994); Frechet, J. M., Science, 263, 1710-1715 (1994); Tomalia, D. A., Naylor, A. M., and Goddard, W. A., Angew. Chem. Intl. Ed. Engl., 29, 138-175 (1990); Biswas P. And Cherayil B. J., J. Chem. Phys., 100(4), 3201-3209 (1994); Kim Y. and Beckerbauer R., Macromolecules, 27, 1968-1971 (1994); Mourey T. et al., Macromolecules, 25, 2401-2406 (1992); Kremers J. A. and Meijer E. W., J. Org. Chem., 59(15), 4262-4266 (1994); van Genderen M. H. P. et al., Rec. T. Chimiques des Pays-Bas, 113(12), 573-574 (1994)).
The nomenclature of dendrimers is described in Newkome, J. Polymer Science, Part A; Polymer Chemistry, 31, (1993), pages 611-651.
For one type of dendrimer, poly(propylene imine), an efficient large scale synthesis has been described (de Brabander-van der Berg, E. M. M. and Meijer, E. W., Angew. Chem. Intl. Ed. Engl., 32-38, 1308 (1993)).
The repetitive reaction sequence involves a Michael addition of two equivalents of acrylonitrile to a primary amino group, followed by hydrogenation of the nitrile groups to primary amine groups. Diaminobutane (DAB) is used as the core molecule. Each complete reaction sequence results in a new xe2x80x9cgenerationxe2x80x9d with a larger diameter and twice the number of reactive functional end groups. For example, starting with diamino butane (DAB), double Michael addition of acrylonitrile yields a species with four cyano groups (DAB-dendr-(CN)4). Catalytic hydrogenation with H2/Raney-Co results in a molecule with four primary amine groups (DAB-dendr-(NH2)4). Repeating this sequence yields dendrimers with 2n cyano or amine end groups, where n is an integer of 2 to 1000, preferably 2 to 100 and more preferably 2 to 20, thus there may be, for example, 8, 16, 32, 64 or 128 such end groups. These end groups may be further reacted or grafted with other molecules to yield the desired surface and/or internal core chemistries.
Similarly ethylene diamine (EDA) may be used instead of diaminobutane (DAB) as the core molecule.
The hyperbranched dendrimeric structure contains primary, secondary and tertiary amines (at various ratios ranging from 0 to 100%). At lower pH values, the primary, secondary and tertiary amines become protonated thereby developing a positive charge. The charges are developed by the interior as well as the surface amine groups. For example, for one type of dendrimer, poly(propylene imine), both the interior tertiary amines as well as the surface primary amines are cationically charged at pH values below 8.
For the purpose of this invention it is necessary that the dendrimeric polymer develop a positive charge at the desired operating pH, and, in particular, this positive charge may be achieved with the end groups. The groups which yield the positive charge may be any suitable groups, for example, amino groups, as for example, primary, secondary, or tertiary amines or quarternized amine functionalities.
Suitably n is chosen such that the dendrimer is readily dispersible in water, and preferably soluble in water. A particularly advantageous subclass of dendrimer has a weight average molecular weight of less than about 50,000. Especially preferred dendrimers have a positive charge of at least 1.5 meq/gram and more preferably at least about 6 meq/gram, most preferably 14 to 19 meq/gram, measured by colloid titration at a pH of 5.
A preferred class of dendrimers are poly(propylene imines) in which the core monomer is a diamino lower alkane of 2 to 10 carbon atoms, preferably 2 to 6 carbon atoms, for example, ethylene diamine (EDA) or diaminobutane (DAB), and the core monomer is reacted with acrylonitrile.
Suitably the dendrimers employed in this invention are prepared by the repetitive reaction sequence involving a Michael addition of two equivalents of acrylonitrile to a primary amine group followed by a hydrogenation of the nitrile groups to primary amines. Diaminobutane and ethylenediamine are preferred core molecules. The end groups are preferably primary amines.
By way of example the molecular weights of the dendrimers used in this invention are 300 and 7,166 Daltons for DAB(PA)4 which is 4-cascade:1,4-diaminobutane-[4]:propylamine and DAB(PA)64 which is 64-cascade:1,4-diaminobutane:(1-aza-butylidene)64 propylamine, respectively and 517 and 1430 Daltons for EDA4 and EDA8, respectively. The respective charge densities at pH 5 are 18.2 meq./gram net and 14.9 meq./gram net for DAB(PA)4 and DAB(PA)64, respectively and 17.0 meq./gram net and 16.4 meq./gram net for EDA(PA)4 and EDA(PA)8 respectively. For comparative purposes, the charges of a typical poly(DADMAC) or branched polyethyleneimine at pH 5 are approximately 5.5 meq./gram net and 5.9 mEq./gram net.
In the process of this invention, the dendrimers are preferably added to the pulp slurry or stock as an aqueous solution before the papermaking stock reaches the paper machine headbox. Ideally, the point of addition is sufficiently before the headbox to enable complete mixing of the polymer into the pulp but after all points of extreme turbulence, such as fan pumps and pressure screens. However, other points of addition may be suitable, either before or after shear locations.
Additionally, the dendrimeric polymers may be added directly to a desired point of addition, such as for example the machine headbox, blend chest, mixing chest, thick stock chests, save-all, or the dilution white water silos/supply lines. Alternatively, the dendrimeric polymers may be mixed directly with the filler slurries or other chemicals prior to their addition to the pulp slurries.
The dendrimeric polymer is added to the pulp or filler slurry in an effective amount. The amount of dendrimeric polymer added can vary depending on several factors, for example, the dendrimeric molecular weight, the dendrimeric surface charge at the operating pH, the pulp used, and the type of surface chemistry. The amount can be determined by those skilled in the art for any particular product or process. However, in general terms, the dendrimeric polymer will be added at a rate between 0.1 and 20 percent by weight based on the weight of oven dried pulp; a preferred embodiment incorporates a range of between 0.1 percent and 5 percent by weight.
The dendrimeric polymers may also be used in conjunction with other papermaking additives for different purposes including improving drainage and retention performance. These additives include various inorganic materials, such as bentonite and alum, and/or organic materials, such as various natural, modified natural, or synthetic polymers which are included in the thin or thick stock for the purpose of improving the drainage and retention process. These can be added, optionally, at locations prior to or after to the addition of the dendrimeric polymer. They may also be added at the same location or variations thereof.